Group III nitride compound semiconductor laser

ABSTRACT

A semiconductor laser comprises a sapphire substrate, an AlN buffer layer, Si-doped GaN n-layer, Si-doped Al 0.1 Ga 0.9 N n-cladding layer, Si-doped GaN n-guide layer, an active layer having multiple quantum well (MQW) structure in which about 35 Å in thickness of GaN barrier layer  62  and about 35 Å in thickness of Ga 0.95 In 0.55 N well layer  61  are laminated alternately, Mg-doped GaN p-guide layer, Mg-doped Al 0.25 Ga 0.75 N p-layer, Mg-doped Al 0.1 Ga 0.9 N p-cladding layer, and Mg-doped GaN p-contact layer are formed successively thereon. A ridged hole injection part B which contacts to a ridged laser cavity part A is formed to have the same width as the width w of an Ni electrode. Because the p-layer has a larger aluminum composition, etching rate becomes smaller and that can prevent from damaging the p-guide layer in this etching process.

[0001] This is a patent application based on Japanese patentapplications No. 2002-063811 and No. 2003-040462, which were filed onMar. 8, 2002 and Feb. 19, 2003, respectively, and which are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a group III nitride compoundsemiconductor laser. Especially, the present invention relates to agroup III nitride compound semiconductor laser having a ridge type of aresonator.

[0004] 2. Description of the Related Art

[0005] A semiconductor laser which has a cladding layer and an activelayer and so on made of a group III nitride compound semiconductor(Al_(x)Ga_(y)In_(1−x−y)N, 0≦x≦1, 0≦y≦1 and 0≦x+y≦1) has been known. Theconventional semiconductor laser is a semiconductor diode havingmulti-layer structure with multiple group III nitride compoundsemiconductor layers, which are formed on a sapphire substrate insequence. A conventional example of the semiconductor diode, disclosedby the same applicant in Japanese Patent Laid-open No. 2000-261105, isshown in FIG. 3.

[0006] A semiconductor laser 900 shown in FIG. 3 has the following eightlayers formed on a sapphire substrate 91 in sequence: a buffer layer 92;an n-layer 93; an n-cladding layer 94; an n-guide layer 95; an activelayer 96 made of multiple quantum well (MQW) layer; a p-guide layer 97;a p-cladding layer 98; and a p-contact layer 99. As shown in FIG. 3, acavity or a resonator part (a ridged resonator cavity of part) A isformed by using, e.g., photoresist and etching, and a positive electrode901 and a negative electrode 902 are formed on the upper surface of thep-contact layer 99 and the etched surface of the n-layer 93,respectively.

[0007] The active layer 96, comprising a multiple quantum well (MQW)layer, is a semiconductor layer which functions as a main layer tooscillate laser. Each carriers (holes and electrons) injected from thepositive electrode 901 and the negative electrode 902 combine in theactive layer 96, that causes laser oscillation. The n-guide layer 95 andthe p-guide layer 97 function to confine carriers into the active layer96. Also, the n-cladding layer 94 and the p-cladding layer 98 functionto confine laser light. And the n-layer 93 and the p-contact layer 99are semiconductor layers which are formed in order that carriers can beinjected smoothly from the negative electrode 902 and the positiveelectrode 901 to the layers existing between the n-cladding layer 94 andthe p-cladding layer 98, respectively.

[0008] In order that the semiconductor laser made of group III nitridecompound semiconductor can oscillate laser efficiently, thecross-section of electric current path of the semiconductor laser is,for example, narrowed by decreasing the contact area of electrodes, orby decreasing the width w of the positive electrode 901. In addition,the above-mentioned Japanese Patent Laid-open No. 2000-261105 suggestsforming a deep ridged hole injection part B. That is, a boundary betweena ridged cavity part A and the ridged hole injection part B is regardedas a boundary between the p-guide layer 97 and the p-cladding layer 98.

[0009] When forming the ridged hole injection part B, however, it is noteasy for all the semiconductor lasers formed on a wafer that a boundarybetween the ridged resonator part A and the ridged hole injection part Bfunctions as a boundary between the p-guide layer 97 and the p-claddinglayer 98. The reason is that each one of group III nitride compoundsemiconductor layers formed on one wafer has different thicknessaccording to the portion on which the layer is formed. So, as disclosesin the above-mentioned official gazette, the applicant of the presentinvention suggests completely etching the p-cladding layer 98 even if aportion of the p-guide layer 97 is etched.

[0010] The thickness of the p-guide layer 97, however, is extremelythin, e.g.,. about 100 nm. So when 200 nm in thickness of p-contactlayer 99 and approximately 500 nm in thickness of p-cladding layer 98are completely etched, the p-guide layer 97 may be damaged considerably,which may deteriorate its device characteristic as a semiconductorlaser.

SUMMARY OF THE INVENTION

[0011] An object of the present invention is to form a ridged carrierinjection part in a ridge type of a group III nitride compoundsemiconductor laser, especially in a process of manufacturing the ridgetype of a group III nitride compound semiconductor laser, so as toobtain a structure which hardly damages a guide layer. Another object ofthe present invention is that the cross sectional shape of oscillatedlaser beam becomes closer to a perfect circle by forming a part ofcladding layer in the ridged cavity part and controlling its thicknessbecomes easy.

[0012] To achieve the above object, a first aspect of the presentinvention is to obtain a group III nitride compound semiconductor lasercomprising a laser cavity and multiple layers which are made of groupIII nitride compound semiconductors and formed on a substrate. The groupIII nitride compound semiconductor laser comprises: a first layer, whichfunctions as a guide layer and actually confines carriers to an activelayer which functions as a main layer oscillating laser; a second layerhaving smaller refractive index compared with the first layer, which isformed above or on the first layer and mainly confines light to theactive layer and the first layer; and a third layer which is formedbetween the first layer and the second layer or formed into the secondlayer and has larger composition of aluminum (Al) in group III elementscompared with the second layer. Here forming the third layer into thesecond layer represents that the second layer comprises two layers andthat the third layer is formed between the upper second layer and thelower second layer. Composition of the upper second layer and the lowersecond layer may be equivalent or not equivalent. In order that thethird layer comes in the scope of the present invention, aluminum (Al)composition of the third layer may be larger than that of at least onelayer of upper and the lower second layers.

[0013] The second aspect of the present invention is that aluminum (Al)composition of group III elements in the second layer is larger thanthat in the first layer.

[0014] The third aspect of the present invention is that the secondlayer functions as a cladding layer.

[0015] The fourth aspect of the present invention is to obtain a groupIII nitride compound semiconductor laser comprising a laser cavity. Thelaser cavity is formed by removing multiple layers, which are made ofgroup III nitride compound semiconductors and formed on a substrate,except the width of the laser cavity part. A carrier injection part isformed contacting to the laser cavity part by removing at least alllayers on the third layer except the area corresponding to the width ofan electrode formed above the second layer.

[0016] The fifth aspect of the present invention is that the electrodeis a positive electrode.

[0017] The sixth aspect of the present invention is that aluminum (Al)composition of the third layer is larger than that of the second layerby 10% or more. That is represented by the formula x3≧x2+0.1 when x3 andx2 (0≦x3, x2≦1) are aluminum (Al) compositions of the third and thesecond layers in all the group III elements. The seventh aspect of thepresent invention is that the third layer is thinner than the firstlayer.

[0018] By forming the third layer having larger aluminum (Al)composition in group III elements compared with that of the second layerbetween the first layer which functions as a guide layer and actuallyconfines carriers to an active layer functioning as a main layer tooscillate laser and the second layer which mainly confine light to theactive layer which functions as a main layer oscillating laser and thefirst layer, the third layer can protect the first layer in an etchingprocess. That is because etching rate of a group III nitride compoundsemiconductor becomes smaller in proportion as the aluminum (Al)composition increases, there are some time during etching the thirdlayer. Accordingly, in a process of forming a ridged carrier injectionpart, a guide layer cannot be damaged even when the production is notuniform. Or because the third layer having larger aluminum (Al)composition of group III elements compared with the second layer isformed between the upper second layer and the lower second layer,forming a part of a cladding layer in the laser cavity part enables iteasier to control thickness of the cladding layer formed in the lasercavity part. As a result, the cross sectional shape of oscillated laserbeam becomes closer to a perfect circle (the first to the fourthaspects). And when a layer of the positive electrode side, or the secondlayer, has p-type conduction, manufacturing process becomes easier (thefifth aspect).

[0019] Because the difference between the aluminum (Al) compositionratios of the second layer and the third layer is 10% or more, etchingrate of the third layer becomes smaller and the etching becomes moreeffective (the sixth aspect). By forming the third layer thinner thanthe first layer, the guide layer may not be damaged withoutdeteriorating characteristics of a laser diode or thickness of thecladding layer formed in the laser cavity part may be controlled easily(the seventh aspect).

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Other objects, features, and characteristics of the presentinvention will become apparent upon consideration of the followingdescription and the appended claims with reference to the accompanyingdrawings, all of which form a part of the specification, and whereinreference numerals designate corresponding parts in the various figures,wherein:

[0021]FIG. 1A is a sectional view of a semiconductor laser 100 inaccordance with the first embodiment of the present invention;

[0022]FIG. 1B is a view showing a structure of the semiconductor laser100 in accordance with the first embodiment of the present invention;

[0023]FIG. 2 is a graph showing a relation of Al composition x (0≦x≦1)in Al_(x)Ga_(1−x)N and etching rate;

[0024]FIG. 3 is a sectional view of a conventional semiconductor laser;and

[0025]FIG. 4 is a view showing a structure of the semiconductor laser200 in accordance with the second embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] The present invention will next be described in detail withreference to embodiments, which should not be construed as limiting theinvention thereto.

[0027]FIG. 1A illustrates a sectional view of a semiconductor laser 100.FIG. 1B illustrates a view showing a structure of the semiconductorlaser 100.

[0028] The semiconductor laser 100 has a sapphire substrate 1, and anAlN buffer layer 2 having a thickness of 50 nm is formed on thesubstrate 1. Alternatively, the buffer layer 2 can be made of GaN, GaInNor AlGaN. On the buffer layer 2, the following layers are formedconsecutively: about 4.0 μm in thickness of silicon (Si) doped galliumnitride (GaN) n-layer 3, having an electron concentration of1×10¹⁸/cm⁻³; 500 nm in thickness of Si-doped Al_(0.1)Ga_(0.9)Nn-cladding layer 4, having an electron concentration of 1×10¹⁸/cm⁻³; 100nm in thickness of Si-doped GaN n-guide layer 5, having an electronconcentration of 1×10¹⁸/cm⁻³; and an active layer 6 having multiplequantum well (MQW) structure in which about 35 Å in thickness of GaNbarrier layer 62 and about 35 Å in thickness of Ga_(0.95)In_(0.05)N welllayer 61 are laminated alternately. And 100 nm in thickness of magnesium(Mg) doped GaN p-guide layer 7, having a hole concentration of5×10¹⁷/cm⁻³, 50 nm in thickness of Mg-doped Al_(0.25)Ga_(0.75)N p-layer8, having a hole concentration of 5×10¹⁷/cm⁻³, 500 nm in thickness of anMg-doped Al_(0.1)Ga_(0.9)N p-cladding layer 9, having a holeconcentration of 5×10¹⁷/cm⁻³, and 200 nm in thickness of Mg-doped GaNp-contact layer 10, having a hole concentration of 5×10¹⁷/cm⁻³, areformed successively thereon. Alternatively, the p-contact layer 10 canbe made of AlGaN or GaInN. Then 5 μm in width of an electrode 11 made ofnickel (Ni) is formed on the p-contact layer 10, and an electrode 12made of aluminum (Al) is formed on the n-layer 3.

[0029] The ridged hole injection part B, which contacts to the ridgedcavity (or resonator) part A of the semiconductor laser 100, is formedto have a width of about 5 μm, which is equal to the width w of the Nielectrode 11. The ridged hole injection part B of the semiconductorlaser 100 comprises the Ni electrode 11, the p-contact layer 10 and thep-cladding layer 9. And the ridged cavity part A does not comprise thep-cladding layer 9.

[0030] A method for manufacturing the semiconductor laser 100 isexplained hereinafter. Each of the layers of the semiconductor laser 100is formed by gaseous phase epitaxial growth, called metal organic vaporphase epitaxy (hereinafter MOVPE). The gases employed in this processwere ammonia (NH₃), a carrier gas (H₂ or N₂) , trimethyl gallium(Ga(CH₃)₃) (hereinafter TMG), trimethyl aluminum (Al(CH₃)₃) (hereinafterTMA), trimethyl indium (In(CH₃)₃) (hereinafter TMI), silane (SiH₄), andbiscyclopentadienyl magnesium (Mg(C₅H₅)₂) (hereinafter CP₂Mg)

[0031] The single crystalline sapphire substrate 1 was placed on asusceptor in a reaction chamber for the MOVPE treatment after its mainsurface ‘a’ was cleaned by an organic washing solvent and heattreatment. Then the sapphire substrate 1 was baked for about 30 min. at1100° C. by H₂ vapor fed into the chamber at a flow rate of 2 L/min.under normal pressure.

[0032] About 50 nm in thickness of AlN buffer layer 2 was formed on thesurface ‘a’ of the baked sapphire substrate 1 under conditionscontrolled by lowering the temperature in the chamber to 400° C.,keeping the temperature constant, and concurrently supplying H₂ at aflow rate of 20 L/min., NH₃ at 10 L/min., and TMA at 18 μmol/min. forabout 90 seconds.

[0033] About 4.0 μm in thickness of Si-doped GaN was formed on thebuffer layer 2, as an n-layer 3 with an electron concentration of 1×10¹⁸cm⁻³, under conditions controlled by keeping the temperature of thesapphire substrate 1 at 1150° C. and concurrently supplying H₂ at a flowrate of 20 L/min., NH₃ at 10 L/min., TMG at 170 μmol/min., and silane(SiH₄) diluted to 0.86 ppm by H₂ at 2 nmol/min.

[0034] About 500 nm in thickness of Si-doped Al_(0.1)Ga_(0.9)N wasformed on the n-layer 3, as an n-cladding layer 4 with an electronconcentration of 1×10¹⁸ cm⁻³, under conditions controlled by keeping thetemperature of the sapphire substrate 1 at 1100° C. and concurrentlysupplying H₂ at a flow rate of 20 L/min., NH₃ at 10 L/min., TMA at 5μmol/min., TMG at 50 μmol/min., and silane (SiH₄) diluted to 0.86 ppm byH₂ at 8 nmol/min.

[0035] About 100 nm in thickness of Si-doped GaN was formed on then-cladding layer 4, as an n-guide layer 5 with an electron concentrationof 1×10¹⁸ cm⁻³, under conditions controlled by keeping the temperatureof the sapphire substrate 1 at 1100° C. and concurrently supplying H₂ ata flow rate of 20 L/min., TMG at 50 μmol/min., and silane (SiH₄) dilutedto 0.86 ppm by H₂ at 8 nmol/min.

[0036] About 35 Å in thickness of GaN was formed on the n-guide layer 5,as a barrier layer 62, concurrently supplying N₂ or H₂, NH₃ and TMG.About 35 Å in thickness of Ga_(0.95)In_(0.05)N was formed on the barrierlayer 62, as a well layer 61, concurrently supplying N₂ or H₂, NH₃, TMGand TMI. Accordingly, four pairs of the well layer 61 and the barrierlayer 62 in total were formed, and an active layer 6 having a multiplequantum well (MQW) structure was obtained.

[0037] About 100 nm in thickness of Mg-doped GaN was formed on theactive layer 6, as a p-guide layer 7, under conditions controlled bykeeping the temperature of the sapphire substrate 1 to 1100° C.,concurrently supplying N₂ or H₂ at a flow rate of 20 L/min., NH₃ at aflow rate of 10 L/min., TMG at 50 μmol/min., and Cp₂Mg at 0.2 μmol/min.

[0038] About 50 nm in thickness of Mg-doped Al_(0.25)Ga_(0.75)N wasformed on the p-guide layer 7, as a p-layer 8, under conditionscontrolled by keeping the temperature of the sapphire substrate 1 to1100° C., concurrently supplying N₂ or H₂ at a flow rate of 20 L/min.,NH₃ at a flow rate of 10 L/min., TMA at 15 μmol/min., TMG at 50μmol/min., and Cp₂Mg at 0.2 μmol/min.

[0039] About 500 nm in thickness of Mg-doped Al_(0.1)Ga_(0.9)N wasformed on the p-layer 8, as a p-cladding layer 9, under conditionscontrolled by keeping the temperature of the sapphire substrate 1 to1100° C., concurrently supplying N₂ or H₂ at a flow rate of 20 L/min.,NH₃ at a flow rate of 10 L/min., TMA at 5 μmol/min., TMG at 50μmol/min., and Cp₂Mg at 0.2 μmol/min.

[0040] About 200 nm in thickness of Mg-doped GaN was formed on thep-cladding layer 9, as a p-contact layer 10, under conditions controlledby keeping the temperature of the sapphire substrate 1 to 1100° C.,concurrently supplying N₂ or H₂ at a flow rate of 20 L/min., NH₃ at aflow rate of 10 L/min., TMG at 50 μmol/min., and Cp₂Mg at 0.2 μmol/min.

[0041] Then, electron rays were uniformly irradiated into the threelayers, or the p-contact layer 10, the p-cladding layer 9, the p-layer 8and the p-guide layer 7, using a reflective electron beam diffractiondevice. The irradiation conditions were set at 10 kV for theaccelerating voltage, 1 μA for the sample current, 0.2 mm/s. for thespeed of the beam scanning, 60 μmφ for the beam aperture, and at 50μTorr vacuum. This irradiation cause to increase hole concentrations ofthree layers, or the p-contact layer 10, the p-cladding layer 9, thep-layer 8 and the p-guide layer 7, the respective hole concentrationsare 5×10¹⁷ cm⁻³, 5×10¹⁷ cm⁻³ and 5×10¹⁷ cm⁻³. As a result, a wafer witha multiple layer structure was obtained.

[0042] About 200 nm in thickness of SiO₂ layer was formed on thep-contact layer 10 by sputtering, and a photoresist layer was laminatedon the SiO₂ layer. The photoresist layer of the other part except theridged hole injection part B, or region X shown in FIG. 1A, was removedby using photolithography. And the SiO₂ layer, which is not covered bythe photoresist layer, was removed by using hydrofluoric acid basedetching solution.

[0043] The portion of the p-contact layer 10 and the p-cladding layer 9,which is not covered by the photoresist layer and the SiO₂ layer, isdry-etched under conditions set at 0.04 Torr vacuum and at 0.44 W/cm²for a high-frequency power, concurrently supplying BCl₃ gas at a flowlate of 10 mL/min. Accordingly, the ridged hole injection part B asshown in FIG. 1B was formed. Then the SiO₂ layer was removed.

[0044] In order to form an electrode 12, region C was formed on aportion of the n-layer 3 as follows. The SiO₂ layer having a thicknessof 200 nm was formed by carrying out sputtering, which was covered by aphotoresist layer. A portion of the photoresist layer, or a portion toform region C, was removed by using photolithography. Then the SiO₂layer, which is not covered by the photoresist layer, was removed byusing hydrofluoric acid based etching solution.

[0045] The p-guide layer 7, the active layer 6, the n-guide layer 5, then-cladding layer 4 and a portion of the n-layer 3, which are not coveredby the photoresist layer and the SiO₂ layer, is dry-etched underconditions set at 0.04 Torr vacuum and at 0.44 W/cm² for ahigh-frequency power, concurrently supplying BCl₃ gas at a flow late of10 mL/min, and then dry-etched by argon (Ar) gas. Accordingly, region Cshown in FIG. 1A was formed. After that the SiO₂ layer was removed.

[0046] Nickel (Ni) was deposited uniformly on the semiconductor laser100. A photoresist layer was laminated on the Ni layer. And afterremoving processes using photolithography and etching, the electrode 11having a width of 5 μm was formed on the p-contact layer 10. Aluminum(Al) was deposited on the n-layer 3, and the electrode 12 was formed.

[0047] Accordingly, the semiconductor laser 100 as shown in FIGS. 1A and1B were obtained. As shown in FIG. 1A, the semiconductor laser 100 hasthe ridged hole injection part B which was formed by etching all thesemiconductor layers, or the p-contact layer 10 and the p-cladding layer9, except the width of the ridged hole injection part B. Although thedevices are not completely uniform, a boundary between the ridged cavitypart A and the ridged hole injection part B are obtained in the p-layer8.

[0048] For comparison, a semiconductor laser 900 was formed as shown inFIG. 3. The semiconductor laser 900 is manufactured in the same processas the semiconductor laser 100, except that the semiconductor laser 900does not have the p-layer 8. In one wafer of the semiconductor laser 900shown in FIG. 3, approximately 10% of the semiconductor laser hasdeteriorated device characteristic, e.g., a semiconductor laser whoseguide layer 97 is largely damaged, and electric current is notadequately narrowed because etching of a cladding layer 98 is notsufficient. On the contrary, in one wafer of the semiconductor laser 100shown in FIG. 1, no semiconductor laser whose guide layer 7 is largelydamaged is found, and there is no problem about narrowing electriccurrent. Thus-obtained semiconductor laser has approximately theequivalent characteristics to an acceptable product of the semiconductorlaser 900 shown in FIG. 3.

[0049]FIG. 2 illustrates a relationship between a composition ratio ofaluminum and an etching rate in the experiment. As shown in FIG. 2, whena difference of composition ratio of aluminum is 10%, etching rate isdifferent for 5%.

[0050]FIG. 4 illustrates a sectional view of a semiconductor laser 200in a second embodiment of the present invention. The semiconductor laser200 shown in FIG. 4 has the same structure as that of the semiconductorlaser 100 in FIG. 1 except that the following layers are laminatedbetween the p-guide layer 7 and the p-contact layer 10 consecutively:about 20 nm in thickness of magnesium (Mg) doped Al_(0.1)Ga_(0.9)N lowerp-cladding layer 910, having a hole concentration of 5×10¹⁷/cm⁻³; about50 nm in thickness of magnesium (Mg) doped Al_(0.25)Ga_(0.75)N p-layer8, having a hole concentration of 5×10¹⁷/cm⁻³; and about 420 nm inthickness of magnesium (Mg) doped Al_(0.1)Ga_(0.9)N upper p-claddinglayer 920, having a hole concentration of 5×10¹⁷/cm⁻³, and that thelower p-cladding layer 910 is formed in the ridged cavity part A. Thesemiconductor laser 200 shown in FIG. 4 is produced by the same methodas that of the semiconductor laser 100 in FIG. 1 except for thefollowing two points: (1) each layer is supplied with raw materialsaccording to the structure of the wafer and is grown epitaxially; and(2) etching time is controlled in order that etching stops at thep-layer 8 formed between the lower p-cladding layer 910 and the upperp-cladding layer 920.

[0051] When the semiconductor laser 200 is oscillated light, the crosssectional shape of the oscillated beam is almost a perfect circle. Onthe contrary, the shape of the oscillated beam of the semiconductorlaser 100 shown in FIG. 1 is a perfect circle with concaves at right andleft of the upper positions of the circle. As described above, becausethe lower p-cladding layer 910 is formed in the ridged cavity part A,the shape of the oscillated laser beam can be close to a perfect circle,and in order to have that beam shape thickness of the loser p-claddinglayer 910 can be easily controlled because there is the p-layer 8 havinga larger aluminum (Al) composition.

[0052] In the first embodiment, a single layer of p-guide layer 7 and asingle layer of p-cladding layer 9 are formed as the first layer and thesecond layer, respectively, and a single layer of p-layer 8 is formed asthe third layer between the first layer and the second layer. And in thesecond embodiment, a single layer of lower p-cladding layer 910 and asingle layer of upper p-cladding layer 920 formed as the second layerand a single layer of p-layer 8 is formed as the third layer between thelower p-cladding layer 910 and the upper p-cladding layer 920.Alternatively, each layer may not have single layer structure.Especially, each layer can have a multi-layer structure (multiple layerstructure), and any layer can be formed between any two of the layersabove described in order to add other function to the device. When thesecond layer has a multi-layer structure, its aluminum composition ofthe layer which comprises most amount of aluminum and actuallydetermines to confine lights, and when the third layer has a multi-layerstructure, its aluminum composition of the layer which comprises mostamount of aluminum and actually determines etching rate, are comparedwith each other.

[0053] Aluminum (Al) composition of the third layer only needs to belarger than that of the second layer. The third layer may be made of,e.g., AlN. When the third layer is adequately thin, electric current canflow by tunneling effect.

[0054] While the invention has been described in connection with whatare presently considered to be the most practical and preferredembodiments, it is to be understood that the invention is not to belimited to the disclosed embodiments, but on the contrary, is intendedto cover various modifications and equivalent arrangements includedwithin the spirit and scope of the appended claims.

What is claimed is:
 1. A group III nitride compound semiconductor laserhaving a laser cavity and multiple layers which are made of group IIInitride compound semiconductors and formed on a substrate comprising: afirst layer functioning substantially as a guide layer to an activelayer; a second layer having smaller refractive index compared with saidfirst layer above or on said first layer; and a third layer which isformed between said first layer and said second layer or into saidsecond layer and has larger composition of aluminum (Al) in group IIIelements compared with the second layer.
 2. A group III nitride compoundsemiconductor laser according to claim 1, wherein said second layer hasa larger aluminum (Al) composition than that of the first layer.
 3. Agroup III nitride compound semiconductor laser according to claim 1,wherein said second layer functions as a cladding layer.
 4. A group IIInitride compound semiconductor laser according to claim 2, wherein saidsecond layer functions as a cladding layer.
 5. A group III nitridecompound semiconductor laser according to claim 1, wherein said lasercavity is formed by removing multiple layers except the width of saidlaser cavity part, and a carrier injection part is formed contacting tosaid laser cavity part by removing at least all layers on said thirdlayer except the area corresponding to the width of an electrode formedabove said second layer.
 6. A group III nitride compound semiconductorlaser according to claim 2, wherein said laser cavity is formed byremoving multiple layers except the width of said laser cavity part, anda carrier injection part is formed contacting to said laser cavity partby removing at least all layers on said third layer except the areacorresponding to the width of an electrode formed above said secondlayer.
 7. A group III nitride compound semiconductor laser according toclaim 3, wherein said laser cavity is formed by removing multiple layersexcept the width of said laser cavity part, and a carrier injection partis formed contacting to said laser cavity part by removing at least alllayers on said third layer except the area corresponding to the width ofan electrode formed above said second layer.
 8. A group III nitridecompound semiconductor laser according to claim 5, wherein saidelectrode is a positive electrode.
 9. A group III nitride compoundsemiconductor laser according to claim 6, wherein said electrode is apositive electrode.
 10. A group III nitride compound semiconductor laseraccording to claim 7, wherein said electrode is a positive electrode.11. A group III nitride compound semiconductor laser according to claim1; wherein aluminum (Al) composition of said third layer is larger thanthat of said second layer by 10% or more.
 12. A group III nitridecompound semiconductor laser according to claim 2, wherein aluminum (Al)composition of said third layer is larger than that of said second layerby 10% or more.
 13. A group III nitride compound semiconductor laseraccording to claim 3, wherein aluminum (Al) composition of said thirdlayer is larger than that of said second layer by 10% or more.
 14. Agroup III nitride compound semiconductor laser according to claim 4,wherein aluminum (Al) composition of said third layer is larger thanthat of said second layer by 10% or more.
 15. A group III nitridecompound semiconductor laser according to claim 1, wherein said thirdlayer is thinner than said first layer.
 16. A group III nitride compoundsemiconductor laser according to claim 2, wherein said third layer isthinner than said first layer.
 17. A group III nitride compoundsemiconductor laser according to claim 11, wherein said third layer isthinner than said first layer.
 18. A group III nitride compoundsemiconductor laser according to claim 12, wherein said third layer isthinner than said first layer.
 19. A group III nitride compoundsemiconductor laser according to claim 13, wherein said third layer isthinner than said first layer.
 20. A group III nitride compoundsemiconductor laser according to claim 14, wherein said third layer isthinner than said first layer.